1. Field of the Invention
The subject invention is generally directed to anti-wear additives and fluid compositions containing anti-wear additives which can effectively reduce wear in ceramic systems and, more particularly, to utilizing monomer compounds that are capable of polymerizing directly on rubbing surfaces under tribological conditions and function as anti-wear additives.
2. Description of the Prior Art
Recently there has been much interest in using ceramic materials in a wide variety of applications which traditionally have utilized metals. Ceramic materials have several advantageous engineering properties. For example, ceramics generally have high hardness, high melting points, low density, low thermal expansion, corrosion resistance, and high resistance to thermal and chemical stresses. Particular ceramic materials which are currently being used or considered for use in tribological applications include the following: alumina (aluminum oxide or Al.sub.2 O.sub.3), zirconia (zirconium oxide or ZrO.sub.2), silicon nitride (Si.sub.3 N.sub.4), silicon carbide (SIC), boron nitride (BN), aluminum nitride (AlN), boron carbide (B.sub.4 C), and beryllia (beryllium oxide BeO). It is envisioned that ceramics will provide useful solutions in advanced propulsion systems, low heat rejection engines, aerospace bearings, turbomachinery, adiabatic diesel engines, high speed roller bearings in gas turbine engines, as well as a wide variety of other systems.
Wear has been defined as the progressive loss of a substance from the operating surface of a body as a result of relative motion at the surface of the body (see, Furey, "Tribology", Encyclopedia of Materials Science & Engineering, Pergamon Press, Oxford, pp. 5145-5157, 1986). When ceramic elements rub together or when a ceramic element rubs against an element made from a different material such as a metal or composite element, wear occurs. The rate of wear tends to increase under harsh temperature and pressure conditions which exist inside ceramic engines, propulsion systems, and the like. In addition to limiting the useful life of the part in which the ceramic is used, wear of ceramics can be costly because the ceramic materials themselves are expensive to produce.
In the prior art, basically four different approaches to lubricating ceramics have been investigated. In the first approach, various materials such as oils, additives, polymers, solid lubricants, and soaps, have been incorporated into ceramics prior to manufacturing the ceramic element so that the resulting ceramic element has a "self-lubricating" property. Since this approach requires incorporation of the materials into the ceramic prior to formation of the ceramic element, it is not useful as a lubrication scheme for lubricating existing state-of-the-art bearings or other components. Moreover, when incorporating the additives into the ceramic, the physical changes to the ceramic produced relative to the ceramic without the additives must be considered. In the second approach, the ceramic elements have been subjected to various surface treatments such as ion sputtering and implantation, or the ceramic elements have had various coatings applied thereto such as metal films, solid lubricants, and polymer films. A particular disadvantage of these surface treatments and film coatings is that they are themselves removed by wear. Hence, surface treatments and film coatings have a finite life and are non-replenishing. In a third approach, conventional soluble anti-wear additives have been added to a fluid carrier (e.g., mineral oil). This approach typically requires a specific reaction with a metal surface to form a low shear film such as iron phosphate, iron sulfide, or iron compounds of unknown composition; therefore, this approach would not be expected to be applicable to the less chemically reactive ceramic materials. In the fourth approach, dispersed solid lubricants such as graphite or molybdenum disulfide in oils have been applied to the ceramic surface. These dispersions have not yielded satisfactory results for a variety of reasons including the fact that high concentrations of the dispersion are generally needed and settling of the dispersion and filter plugging occurs.
In short, none of the four approaches discussed above have been very successful in lubricating ceramics and a new approach to reducing ceramic wear which overcomes the difficulties of the prior art would be most beneficial.